1. Field of the Invention
The invention relates to communications; in particularly, packet transmission scheduling for cellular networks and to a packet transmission scheduling system having such efficient packet transmission scheduling functionality.
2. Description of the Prior Art
There are problems associated with mobility handling in packet switched networks having scheduling systems, especially in packet switched cellular mobile communication systems, and particularly Code Division Multiple Access systems (CDMA) like Universal Mobile Telecommunication Systems (UMTS) or Time Division Multiple Access systems (TDMA) like General Packet Radio Systems (GPRS).
As known, in packet switching networks the task of multiplexing essentially reduces to the task of ordering packets and to then send them serially over a shared link. This process of serialization is referred to as scheduling. For certain network links, especially for wireless links a certain amount of pre-given link characteristics have to be applied to protocol data units (PDU) to be transmitted, that is traditionally addressed by a segmentation of layer-3-PDU performed by a layer-3 scheduler, whereas a lower so called MAC-scheduler (medium access control-scheduler) provides the medium access control including the provision of respective transport blocks for a transmission data flow via the Physical-layer (PHY-layer).
However, recent scheduling methods consist of independent scheduling systems of each cell within a radio network controller (RNC), such as of an UMTS based network for example, and accordingly, there is in general no interconnection between these schedulers. In case of a handover procedure for example, i.e. when a mobile station is moving out of the coverage of a current cell and hence, has to be handed over towards a new cell which is better able to serve the data flow of the mobile station, within such a recent data transmission scheduling scheme the handover procedure may be described as follows:
Since there is no interconnection between the scheduling systems of different cells, the handover procedure has to be performed by means of a central instance. In packet switched networks this is usually the Service Gateway Sub Node (SGSN).
During the handover procedure the SGSN sends the layer-3 PDUs towards the scheduler of the new or target cell starting with the last layer-3 PDU, which was not completely transmitted by the old cell scheduler.
Then, the target cell scheduler starts to transmit beginning with the first transport block that is segmented from the layer-3 PDU.
By receiving of the transport blocks from this PDU the mobile station has to discard the transport blocks from the PDU that was not entirely transmitted by the scheduler of the old cell.
Since the handover process is performed over a central instance, there is a significant delay for establishing the data transmission in the new cell resulting in an non-continuous data flow with regard to the mobile station and hence for its user. Moreover since the new cell scheduler is starting its transmission always with the first transport blocks from the current layer-3 PDU there is a waste of resources, too.
Since the scheduling status is not transferred to the scheduler serving the new cell, the fact that the flow was potentially backlogged, i.e. it has received less service in the past than it has requested, is not taken into account by the new scheduler.